manure was applied to the field a year or two prior to the winterkill problems.that is the most likely source of the nutrient problem even though the water must flow through about 100 yards of grass before reaching the water.
All we have had is our local watershed manager use a meter to test DO oxygen levels After we thought we had done a good job of keeping the ice clear. He found DO under 1 at about 12 ft if I remember correctly.
Winter-ice represents a variable in which I have zero experience; but I'll theorize that the spreading of manure within the watershed - coupled with inadequate aeration/destratification - both played pivotal roles in the subsequent fish-kills.
I'll now grab my popcorn and watch from the cheering section.
